Conventionally, rolls having an outer layer of grain cast iron have been used in the latter strand of a finishing train of a hot strip mill. When grain rolls meet abnormal draw rolling, the grain rolls suffer from little seizing of rolled material as well as little occurring or extending of cracks, because the grain roll, in general, is excellent in seizing resistance. However, the grain roll is fairly inferior in wear resistance to a compound roll having an outer layer of high-speed steel material, which has is recently come to be widely used. Although the high-speed steel roll is excellent in wear resistance, such roll is susceptible to seizing of rolled material by abnormal draw rolling, resulting in ocurring or extending cracks due to the stress concentration at seizing portion by high pressure from back-up rolls or the rolled material.
It has been known that crystallization or precipitation of hard carbides such as MC, M.sub.2 C, etc is effective for improving wear resistance of a roll. Also, it has been known that crystallization of graphites which as a solid lubricant can improve seizing resistance of a roll. However, V, Mo, W which are hard carbide-forming elements are also white cast iron-forming elements. Therefore, it has been unable to crystallizing a suitable amount of graphite in high-speed steel roll containing a large amount of these white cast iron-forming elements to allow hard carbides and graphites to coexist.
To solve this problem, various attempts have been made. JP-B-60-23183 discloses a tough, wear-resistant roll for rolling mill made of a cast ironing a composition consisting of 2.2-2.9% of C, 0.8-1.5% of Si, 0.5-1.0% of Mn, 0.1% or less of P, 0.1% or less of S, 3.8-4.8% of Ni, 1.7-2.5% of Cr, 0.4-1.0% of Mo and balance substantially consisting of Fe. The roll has a structure comprising a matrix of martensite and/or bainite, carbides having an area ratio of 10-30% and graphites having an area ratio of 0.5-3%. The Shore hardness of the roll is 70-85. The roll of JP-B-60-23183, however, is insufficient in in wear resistance because of a small amount of carbides.
JP-A-61-26758 discloses a seizing-resistant compound roll having an outer layer of a composition consisting, by weight, of 1.0-2.0% of C, 0.2-2.0% of Si, 0.5-1.5 of Mn, 3.0% or less of Ni, 2-5% of Cr, 3-10% of Mo, 4.0% or less of V 0.1-0.6% of S and balance substantially consisting of Fe. In this roll, seizing resistance is intended to be improved by forming MnS, etc. However, it is now known that graphite is more effective than MnS for improving seizing resistance.
JP-A-2-30730 disclose a wear-resistant cast iron for use in a roll for hot or cold rolling, having a composition consisting, by weight, of 2.5-4.0% of C. 2.0-5.0% of Si, 0.1-1.5% of Mn, 3-8% of Ni, 7% or less of Cr, 4-12% of Mo, 2-8% of V and balance consisting of Fe and impurities. This cast iron contains graphites and hard carbides such as MC, M.sub.2 C, M.sub.6 C, M.sub.4 C.sub.3, etc. in an area ratio of 20% or less. In this cast iron, an Si-containing inoculant such as Fe--Si alloy, etc. is added into a melt of a casting material to crystallize graphite. Specifically, in Example 1, an Fe--Si alloy is inoculated into a melt in a ratio of 0.3% based on Si to obtain a casting product in which the area ratio of graphite is 2% and the ratio of the area of hard carbides to the area of total carbides is 85%.
In case of a high-speed steel roll, however, it has been found that graphite does not crystallize in a sufficient amount by the inoculation method disclosed in JP-A-2-30730, because a sufficient effect of inoculation cannot be obtained by merely adding an inoculant into a melt at tapping of the melt.
It is difficult to insure a sufficient crystallization of graphite in an outer layer, namely, in a high-speed steel roll disclosed in WO 88/07594, namely, a wear-resistant compound roll comprising an outer layer of an iron-based alloy consisting, by weight, of 1.5-3.5% of C, 0.3-3.0% of Si, 0.3-1.5% of Mn, 2-7% of Cr, 9% or less of Mo, 20% or less of W, 3-15% of V and balance substantially consisting of Fe, and a steel shaft metallurgically bonded to the outer layer; and produced by a shell casting method.
Accordingly, an object of the present invention is to solve the above problem and provide a graphite-containing, high-speed steel roll for hot rolling excellent in both wear resistance and seizing resistance.